qdev: make properties array "const"

Constify all accesses to qdev properties, except for the
ObjectPropertyAccessor itself.  This makes it possible to place them in
read-only memory, and also lets Rust bindings switch from "static mut"
arrays to "static"; which is advantageous, because mutable statics are
highly discouraged.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

system/qdev-monitor.c

@@ -751,7 +751,7 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
 
 #define qdev_printf(fmt, ...) monitor_printf(mon, "%*s" fmt, indent, "", ## __VA_ARGS__)
 
-static void qdev_print_props(Monitor *mon, DeviceState *dev, Property *props,
+static void qdev_print_props(Monitor *mon, DeviceState *dev, const Property *props,
                              int indent)
 {
     if (!props)